以下面这个命令启动：
1 java -server -Xms15g -Xmx15g -jar /app.jar
应用程序的内存占用会是多少？
周四的时候，同事突然问了一个问题，为什么设置了Xms=Xmx=15g之后，应用程序的内存占用远远小于Xms？
具体启动命令如下：
1 2 3 4 5 6 7 8 9 10 //为了方便查看，手动换行 java -server -Xms15g -Xmx15g -XX:+UseConcMarkSweepGC -XX:+CMSParallelRemarkEnabled -XX:+UseCMSInitiatingOccupancyOnly -XX:CMSInitiatingOccupancyFraction=77 -XX:+ScavengeBeforeFullGC -XX:+CMSScavengeBeforeRemark -Djava.security.egd=file:/dev/./urandom -jar /app.jar --spring.profiles.active=prod
下面看一下真实的内存占用
先从系统层面看：
top:
free:
从系统层面可以看到，应用内存占用远小于Xms，难道是JVM参数没有生效？
下面借助arthas 看一下JVM相关信息
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 RUNTIME --------------------------------------------------------------------------------------------------------------------------------------------------------------------- MACHINE-NAME 7@authserver-5fc5c7c775-9fs2m JVM-START-TIME 2020-04-15 00:04:27 MANAGEMENT-SPEC-VERSION 1.2 SPEC-NAME Java Virtual Machine Specification SPEC-VENDOR Oracle Corporation SPEC-VERSION 1.8 VM-NAME Java HotSpot(TM) 64-Bit Server VM VM-VENDOR Oracle Corporation VM-VERSION 25.111-b14 INPUT-ARGUMENTS -javaagent:/var/agent/pinpoint-agent/pinpoint-bootstrap.jar -Xms15g -Xmx15g -XX:+UseConcMarkSweepGC -XX:+CMSParallelRemarkEnabled -XX:+UseCMSInitiatingOccupancyOnly -XX:CMSInitiatingOccupancyFraction=77 -XX:+ScavengeBeforeFullGC -XX:+CMSScavengeBeforeRemark -Djava.security.egd=file:/dev/./urandom CLASS-PATH /app.jar:/var/agent/pinpoint-agent/pinpoint-bootstrap.jar BOOT-CLASS-PATH /usr/java/jre1.8.0_111/lib/resources.jar:/usr/java/jre1.8.0_111/lib/rt.jar:/usr/java/jre1.8.0_111/lib/sunrsasign.jar :/usr/java/jre1.8.0_111/lib/jsse.jar:/usr/java/jre1.8.0_111/lib/jce.jar:/usr/java/jre1.8.0_111/lib/charsets.jar:/usr /java/jre1.8.0_111/lib/jfr.jar:/usr/java/jre1.8.0_111/classes LIBRARY-PATH /usr/java/packages/lib/amd64:/usr/lib64:/lib64:/lib:/usr/lib --------------------------------------------------------------------------------------------------------------------------------------------------------------------- CLASS-LOADING --------------------------------------------------------------------------------------------------------------------------------------------------------------------- LOADED-CLASS-COUNT 21926 TOTAL-LOADED-CLASS-COUNT 21944 UNLOADED-CLASS-COUNT 18 IS-VERBOSE false --------------------------------------------------------------------------------------------------------------------------------------------------------------------- COMPILATION --------------------------------------------------------------------------------------------------------------------------------------------------------------------- NAME HotSpot 64-Bit Tiered Compilers TOTAL-COMPILE-TIME 366850(ms) --------------------------------------------------------------------------------------------------------------------------------------------------------------------- GARBAGE-COLLECTORS --------------------------------------------------------------------------------------------------------------------------------------------------------------------- ParNew 1797/78247(ms) [count/time] ConcurrentMarkSweep 3/460(ms) [count/time] --------------------------------------------------------------------------------------------------------------------------------------------------------------------- MEMORY-MANAGERS --------------------------------------------------------------------------------------------------------------------------------------------------------------------- CodeCacheManager Code Cache Metaspace Manager Metaspace Compressed Class Space ParNew Par Eden Space Par Survivor Space ConcurrentMarkSweep Par Eden Space Par Survivor Space CMS Old Gen --------------------------------------------------------------------------------------------------------------------------------------------------------------------- MEMORY --------------------------------------------------------------------------------------------------------------------------------------------------------------------- HEAP-MEMORY-USAGE 15992750080(14.9 GiB)/16106127360(15.0 GiB)/15992750080(14.9 GiB)/2393542120(2.2 GiB) [committed/init/max/used] NO-HEAP-MEMORY-USAGE 255647744(243.8 MiB)/2555904(2.4 MiB)/-1(-1 B)/247922192(236.4 MiB) [committed/init/max/used] PENDING-FINALIZE-COUNT 0 --------------------------------------------------------------------------------------------------------------------------------------------------------------------- OPERATING-SYSTEM --------------------------------------------------------------------------------------------------------------------------------------------------------------------- OS Linux ARCH amd64 PROCESSORS-COUNT 16 LOAD-AVERAGE 5.08 VERSION 4.4.0-141-generic --------------------------------------------------------------------------------------------------------------------------------------------------------------------- THREAD --------------------------------------------------------------------------------------------------------------------------------------------------------------------- COUNT 256 DAEMON-COUNT 103 PEAK-COUNT 263 STARTED-COUNT 41467 DEADLOCK-COUNT 0 --------------------------------------------------------------------------------------------------------------------------------------------------------------------- FILE-DESCRIPTOR --------------------------------------------------------------------------------------------------------------------------------------------------------------------- MAX-FILE-DESCRIPTOR-COUNT 1048576 OPEN-FILE-DESCRIPTOR-COUNT 811 Affect(row-cnt:0) cost in 6 ms.
注意HEAP-MEMORY-USAGE部分
1 HEAP-MEMORY-USAGE 15992750080(14.9 GiB)/16106127360(15.0 GiB)/15992750080(14.9 GiB)/2393542120(2.2 GiB) [committed/init/max/used]
从这里可以看到JVM参数是生效的，那为什么内存占用不是15g？
